1. Field of the Invention
This invention relates to a hydraulic circuit suitable for use with power cylinders on construction machines such as hydraulic power shovels and the like.
2. Description of the Prior Art
In the hydraulic circuit of power cylinder which holds a load with oil pressure in an oil chamber on the side of the cylinder head, it has been known to provide a logic valve between the oil chamber on the side of the cylinder head and a direction control valve in order to prevent the load from dropping spontaneously by contraction of the cylinder due to oil leaks when the direction control valve is in neutral position, and to add a damping valve to the logic valve in order to prevent the hunting which would occur when contracting the cylinder for lowering the load, as proposed, for example, in Japanese Laid-Open Utility Model Application No. 61-186804.
With the above-described conventional arrangement, when contracting the cylinder, a pilot pressure change-over valve is switched into a communicating position by the pressure in an oil pressure supply duct leading to an oil chamber on the rod side of the cylinder, thereby draining the oil in a spring chamber of the logic valve into the tank to open the logic valve, and then draining the oil in the oil chamber on the side of the cylinder head to the tank through the logic valve and the direction control valve to contract the cylinder. Therefore, in the initial stage of contraction, the cylinder is contracted momentrarily irrespective of the degree of opening of the direction control valve spool to an extent corresponding to the opening stroke volume (the amount of oil drained to the tank from the spring chamber) of the logic valve poppet, creating a dangerous situation of dropping the load momentarily.
Besides, when extending the cylinder, the pilot change-over valve is in blocking position and the logic valve is in locked state, so that it is necessary to provide a check valve parallel with the logic valve to supply pressure to the oil chamber on the side of the cylinder head. This check valve has to be of a large diameter as the flow rate of the pressurized oil to the oil chamber on the side of the cylinder head is greater than to the oil chamber on the rod side. In addition, the logic valve with the adjuvant damping cylinder makes the construction complicated while the provision of the check valve of a large diameter for the logic valve increases the number of parts, which will be eventually reflected by an increase in cost.
Further, the pilot change-over valve which brings the spring chamber of the logic valve into and out of communication with the tank is of the spool type which inevitably entails oil leaks even in blocking position, which might cause spontaneous contraction of the cylinder and drop of the load.